H4-isozyme of lactate dehydrogenase in a solution of sodium C chloride--6. The effect of oxalate and oxamate on the molecular weight

Some differences between the effects of oxalate and oxamate were observed. The oxalate formed the stable tetramers and some aggregates, while the oxamate formed the mixture of dimers, tetramers, octamers and aggregates. The ratios between these molecular forms were different as the oxamate concentration was changed. In the coexistence of inhibitors and pyruvate, pyruvate may act to decrease the molecular weight and to increase the amount of aggregates. The lower molecular weight may be caused by the existence of the active complex, Ed N S1. The effect of the exchange between pyruvate and oxamate bound with enzyme complexes may be expected.     

Inhibition of 15-hydroxyprostaglandin dehydrogenase by papaverine.

Papaverine was found to inhibit NAD⁺-linked 15-hydroxyprostaglandin dehydrogenase partially purified from guinea pig lung. The inhibition was noncompetitive with prostaglandin E₂, uncompetitive with NAD⁺, and reversible. The Ki was calculated to be 26 μM. Papaverine also inhibited the enzyme from swine lung, chicken and dog heart, and rat and dog kidney. The inhibitory effects of papaverine on the 15-hydroxyprostaglandin dehydrogenase were compared with those on cyclic AMP phosphodiesterases in these tissues.     

Inhibition of mammalian succinate dehydrogenase by carboxins.

Carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide), a systemic fungicide, is known to inhibit the oxidation of succinate selectively in a variety of fungi and bacteria. Except for one report, the action of carboxin and of structurally related oxathiin derivatives on mammalian succinate dehydrogenase have not been investigated, however. In the present study, the inhibition of succinate oxidation by a number of carboxin derivatives have been studied using inner membrane preparations, purified particulate preparations (Complex II), and soluble preparations from beef heart. The site of action of carboxins has been studied by using a variety of electron acceptors. It has been concluded that carboxins inhibit mammalian succinate dehydrogenase by reacting at the same site as thenoyltrifluoroacetone but are effective at far lower concentrations. The maximal extent of inhibition by carboxins varies with the type of catalytic assay used and, in general, parallels the extent of inactivation brought about by cyanide, as if both types of agents modified the environment of an iron-sulfur component in the enzyme, presumably the superoxidized (HiPIP) Fe-S cluster.     

Inhibition of pyruvate dehydrogenase complex by moniliformin.

The mechanism for the inhibition of pyruvate dehydrogenase complex from bovine heart by moniliformin was investigated. Thiamin pyrophosphate proved to be necessary for the inhibitory action of moniliformin. The inhibition reaction was shown to be time-dependent and to follow first-order and saturation kinetics. Pyruvate protected the pyruvate dehydrogenase complex against moniliformin inactivation. Extensive dialysis of the moniliformin-inactivated complex only partially reversed inactivation. Moniliformin seems to act by inhibition of the pyruvate dehydrogenase component of the enzyme complex and not by acting on the dihydrolipoamide transacetylase or dehydrogenase components, as shown by monitoring the effect of moniliformin on each component individually. On the basis of these results, a suicide inactivator mechanism for moniliformin on pyruvate dehydrogenase is proposed.     

Inhibition of Trypanosoma cruzi alpha-hydroxyacid dehydrogenase-isozyme II by N-isopropyl oxamate and its effect on intact epimastigotes

The effect of N-isopropyl oxamate on the activity of alpha-hydroxyacid dehydrogenase-isozyme II (HADH-isozyme II) from Trypanosoma cruzi was investigated. The kinetic studies showed that this substance was a competitive inhibitor of this isozyme. The attachment of the nonpolar isopropylic branched chain to the nitrogen of oxamate increased 12-fold the affinity of N-isopropyl oxamate for the active site of T. cruzi HADH-isozyme II. N-isopropyl oxamate was a selective inhibitor of HADH-isozyme II, since other T. cruzi dehydrogenases were not inhibited by this substance. Since HADH-isozyme II participates in the energy metabolism of T. cruzi, a trypanocidal effect can be expected with inhibitors of this isozyme. However, although it was not possible to detect any trypanocidal activity with N-isopropyl oxamate when the ethyl ester was tested as a possible trypanocidal prodrug, the expected trypanocidal effect was obtained, comparable to that obtained with nifurtimox and benznidazole.     

Inhibition of fucosyltransferase V by a GDP-Azasugar.

A GDP-azasugar conjugate was synthesized starting from an enzymatically obtained phosphorylated azasugar. It inhibits human fucosyltransferase V at micromolar concentrations, which is discussed in terms of transition state analogy.     

Inhibition of malate dehydrogenase by thyroxine and structurally related compounds.

The inhibition of pig heart mitochondrial malate dehydrogenase (L-malate: NAD+ oxidoreductase, EC 1.1.1.37) by the thyroxine and structurally related compounds was studied to resolve a longstanding question about the exact nature of the inhibition. Thyroxine, in freshly prepared solution, was found to be a "pure" competitive inhibitor relative to the nucleotide cofactor. Upon standing in diffuse daylight, solutions of thyroxine showed increased ability to inhibit the enzyme, presumably as a result of oxidation of enzyme sulfhydryl groups by free iodine that is released photochemically. This behavior probably accounts for earlier reports of irreversible inactivation by thyroxine. Comment is made on the implications of these findings to the mechanism of thyroid hormmone action.     

Inhibition of pyruvate dehydrogenase and pyruvate dehydrogenase phosphate phosphatase by glyoxylate

The overall reaction catalyzed by the pyruvate dehydrogenase complex from rat epididymal fat tissue is inhibited by glyoxylate at concentrations greater than 10 μm. The inhibition is competitive with respect to pyruvate; K_i was found to be 80 μm. Qualitatively similar results were observed using pyruvate dehydrogenase from rat liver, kidney, and heart. Glyoxylate also inhibits the pyruvate dehydrogenase phosphate phosphatase from rat epididymal fat, with the inhibition being readily detectable using 50 μm glyoxylate. These effects of glyoxylate are largely reversed by millimolar concentrations of thiols (especially cysteine) because such compounds form relatively stable adducts with glyoxylate. Presumably these inhibitions by low levels of glyoxylate had not been previously observed, because others have used high concentrations of thiols in pyruvate dehydrogenase assays. Since the inhibitory effects are seen with suspected physiological concentrations, it seems likely that glyoxylate partially controls the activity of pyruvate dehydrogenase in vivo.     

Inhibition of alcohol dehydrogenase by bismuth

Bismuth compounds have been widely used for the treatment of ulcers and Helicobacter pylori infection, and enzyme inhibition was thought to be crucial for bismuth anti-microbial activity. We have investigated the interaction of colloidal bismuth subcitrate (CBS) with alcohol dehydrogenase and our results demonstrate that bismuth can effectively inhibit the enzyme. Kinetic analysis revealed that CBS acted as a non-competitive inhibitor of yeast alcohol dehydrogenase. Both UV-vis and fluorescence data show that interaction of CBS with the enzyme exhibits biphasic processes. Bismuth can replace only half of Zn(II) from the enzyme (i.e., about one Zn(II) per monomer). Surprisingly, binding of CBS also induces the enzyme dissociation from its native form, tetramer into dimers. The inhibition of Bi(III) on the enzyme is probably due to its direct interference with the zinc sites. This study is likely to provide an insight into the mechanism of action of bismuth drugs.